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We report on SRG/eROSITA, ZTF, ASAS-SN, Las Cumbres, NEOWISE-R, and SwiftXRT/UVOT observations of the unique ongoing event AT 2019avd, located in thenucleus of a previously inactive galaxy at $z=0.029$. eROSITA first observed AT2019avd on 2020-04-28 during its first all sky survey, when it was detected asan ultra-soft X-ray source ($kT\sim 85$ eV) that was $\gtrsim 90$ timesbrighter in the $0.2-2$ keV band than a previous 3$\sigma$ upper flux detectionlimit (with no archival X-ray detection at this position). The ZTF opticallight curve in the $\sim 450$ days preceding the eROSITA detection is doublepeaked, and the eROSITA detection coincides with the rise of the second peak.Follow-up optical spectroscopy shows the emergence of a Bowen fluorescencefeature and high-ionisation coronal lines ([\ion{Fe}{X}] 6375 {\AA},[\ion{Fe}{XIV}] 5303 {\AA}), along with persistent broad Balmer emission lines(FWHM$\sim 1400$ km s$^{-1}$). Whilst the X-ray properties make AT 2019avd apromising tidal disruption event (TDE) candidate, the optical properties areatypical for optically selected TDEs. We discuss potential alternative originsthat could explain the observed properties of AT 2019avd, such as a stellarbinary TDE candidate, or a TDE involving a super massive black hole binary.

AT 2019avd: A novel addition to the diverse population of nuclear transients